Such a vacuum tube is put into an active state by filling a discharge gas for electric discharge into a reduced-pressure vessel and by causing the electric discharge to occur by applying the electric power from internal or external electrodes. As the kinds of electric discharges, there are various types such as arc discharge, glow discharge, and electron beam emission, but they are the same in that the operation is carried out by supplying the electric power to the reduced-pressure vessel. There are various methods for applying the electric power, various electrode layouts, and so on depending on uses. Generally, there are also various types such that electrons are emitted from a hot cathode, electrons are extracted by applying an electric field to an electron emitting electrode, and so on. Dc power or ac power is given as the electric power and selection is made according to the combination of electrodes with the electric power together with uses.